co2 conversion to renewable fuels · integrated masters in environmental engineering 2014/2015 co 2...

INTEGRATED MASTERS IN ENVIRONMENTAL ENGINEERING

2014/2015

CO2 CONVERSION TO RENEWABLE FUELS

Nuno Nascimento Ciravegna da Fonseca

Dissertation submitted for the degree of:

MASTER IN ENVIRONMENTAL ENGINEERING

President of the Jury: ___________________________________________________________

Supervisor: Adrián M.T. Silva (Principal Investigator, Department of

Chemistry, Faculty of Engineering, University of Porto)

Co-Supervisor: Luisa M. Pastrana Martínez (Post-Doc Investigator,

Department of Chemistry, Faculty of Engineering, University of Porto)

Co-Supervisor: Joaquim L. Faria (Associate Professor, Department of

Chemistry, Faculty of Engineering, University of Porto)

Department of Chemical Engineering

Faculty of Engineering - University of Porto

February 2015

CO2 conversion to renewable fuels I

Agradecimentos

Esta página é dedicada às pessoas merecedoras de toda a minha admiração e

reconhecimento que, pelas mais variadas formas, deram o seu contributo na minha vida

e no meu percurso académico.

Quero agradecer, em primeiro lugar, aos meus orientadores Doutor Adrián Silva,

Doutora Luisa Martínez e Professor Doutor Joaquim Faria, pela oportunidade que me

deram de realizar este trabalho inovador. Quero deixar um agradecimento muito

especial à Doutora Luisa Martínez, pela sua simpatia, disponibilidade e o apoio total

prestado durante o projeto pois foi o pilar fundamental para o desenvolvimento desta

dissertação. Ao Dr. Carlos Sá do CEMUP (Centro Materiais da Universidade do Porto)

pela assistência técnica prestada com as análises de SEM e XPS.

Agradeço aos investigadores do Laboratório de Catálise de Materiais (LCM): Dra.

Salomé Soares, Dr. Sérgio Torres, Dra. Carla Fonseca, Dra. Alexandra Gonçalves, Dra.

Rita Ribeiro, Dra. Cláudia Silva e Dra. Marina González pela simpatia transmitida e

disponibilidade. Também quero dar o meu agradecimento aos colegas bolseiros

Ricardo, Raquel, Maria José S., Maria José L., Lucília, Teresa, Nuno, Diogo, Tânia,

Marta, Patrícia e Carla pela simpatia e acompanhamento.

Fora do campo académico, quero agradecer aos meus Pais pelo apoio

incondicional e incentivo prestado ao longo da minha vida académica, à minha irmã

Filipa, ao meu irmão Miguel, à minha madrinha Gabi e ao meu padrinho Pedro. Para

finalizar a esfera familiar, quero agradecer profundamente aos meus avós maternos por

todo o amor, dedicação e valores que me conseguiram transmitir, guardo-vos para

sempre no meu coração.

Quero agradecer à minha querida namorada, Cristiana, por todo o carinho,

paciência e apoio que me deu durante esta longa viagem académica. Aos meus amigos

de infância, Paulo, Daniel, Renato e Mariana por todos os inúmeros momentos de

convívio. Ao meu amigo Alain, pelos fantásticos momentos vividos e pela sua ímpar

visão que me transmitiu. Aos meus amigos do polo-aquático, onde incorporei um

verdadeiro espírito de equipa. A todos meus colegas de curso com quem tive a

oportunidade de conviver durante o percurso académico, em particular ao meu amigo

Samuel.

CO2 conversion to renewable fuels II

CO2 conversion to renewable fuels III

Abstract

One solar energy based technology to recycle carbon dioxide (CO2) into

transportable hydrocarbon fuels (i.e., a solar fuel) would help to reduce atmospheric

CO2 levels and partially fulfill energy demands within the present hydrocarbon based

fuel infrastructure.

Graphene oxide (GO) has stimulated the interest in the design of high-

performance photocatalysts with the aim to enhance the photoefficiency of the

semiconductors under visible light conditions. On the other hand, the presence of co-

catalysts such as copper plays a crucial role in semiconductor-based photocatalyst,

increasing CO2 reduction.

This work describes the photocatalytic reduction of CO2, in aqueous phase, into

hydrocarbons with high added value (e.g., methanol and ethanol). Composite

photocatalysts prepared from TiO2 and GO by the liquid phase deposition method were

used. Copper-loaded catalysts from different copper precursors were synthesized. The

photocatalysts were characterized by scanning electron microscopy (SEM), diffuse

reflectance ultraviolet-visible spectroscopy (DRUV) and X-ray photoelectron

spectroscopy (XPS). The experiments were carried out in alkaline solution (0.2 M

NaOH) and under ultraviolet-visible (UV/Vis) irradiation (> 350 nm).

The results revealed methanol and ethanol as the main reduction products. The

combination of GO with TiO2 exhibited higher photocatalytic activity for methanol

production than TiO2 alone. The high efficiency of this composite was attributed to the

optimal assembly between the TiO2 nanoparticles and GO sheets, since GO material can

act simultaneously as electron acceptor and donor, thus suppressing charge

recombination.

TiO2 loaded with Cu, using Cu(NO3)2 as copper precursor (TiO2_Cu(NO3)2),

showed the best performance for both methanol and ethanol production (23.9 and 124

µmol g-1

h-1

, respectively). The copper(I) species detected in the photocatalysts also

promoted a decrease of the charge recombination phenomenon. The mechanism

assumes the formation of formic acid (HCOOH) and oxalic acid (HOOC-COOH)

adsorbed on the surface as the primary products.

Keywords: CO2 photoreduction, solar fuels, TiO2, graphene oxide, copper.

CO2 conversion to renewable fuels IV

CO2 conversion to renewable fuels V

Resumo

As tecnologias baseadas na energia solar, capazes de reciclar o dióxido de carbono

(CO2) e transformá-lo num combustível facilmente transportável (i.e., um combustível

solar), ajudariam a reduzir os níveis de CO2 atmosféricos e a compensar parcialmente a

procura de energia no atual sector de combustíveis à base de hidrocarbonetos.

O óxido de grafeno (GO-graphene oxide) tem estimulado o interesse na área do

desenho de fotocatalisadores de elevado rendimento, de modo a evidenciar a

foto-eficiência de semicondutores sob condições de luz visível. Por outro lado, a

presença de co-catalisadores, tal como o cobre, desempenham um papel fundamental

nos fotocatalisadores à base de semicondutores, aumentando assim a redução do CO2.

Este trabalho descreve a redução fotocatalítica do CO2 aquoso em hidrocarbonetos

de elevado valor acrescentado (ex. metanol e etanol), utilizando H2O como agente

redutor. Os fotocatalisadores foram preparados utilizando TiO2 e GO pelo método de

deposição em fase líquida. Os catalisadores modificados com cobre foram sintetizados

utilizando diferentes precursores de cobre. Os fotocatalisadores foram caracterizados

por microscopia eletrónica de varrimento (SEM, scanning electron microscopy),

espectroscopia de refletância difusa no ultravioleta-visível (DRUV, diffuse reflectance

ultraviolet-visible spectroscopy) e espectroscopia de fotoeletrões de raios-X (XPS, X-

ray photoelectron spectroscopy). As experiências foram realizadas em solução alcalina

(0.2 M NaOH) e sob irradiação ultravioleta-visível (> 350 nm).

Os resultados revelaram que o metanol e o etanol foram os principais produtos de

redução do CO2. A combinação do GO com TiO2 exibiu maior atividade fotocatalítica

na produção de metanol que o TiO2. A elevada eficiência deste material compósito foi

atribuída ao ajuste ideal entre as nanopartículas de TiO2 e as folhas de GO, permitindo

ao material atuar simultaneamente como aceitador e dador de eletrões, evitando assim a

recombinação de carga.

O TiO2 modificado com cobre (TiO2_Cu(NO3)2), sintetizado a partir do percursor

Cu(NO3)2, demonstrou ter o melhor desempenho na produção de metanol e etanol (23.9

e 124 µmol g-1

h-1

, respectivamente). O cobre(I) detetado nos fotocatalisadores, também

promoveu uma diminuição do fenómeno de recombinação de carga. O mecanismo

assume a formação de ácido fórmico (HCOOH) e ácido oxálico (HOOC-COOH),

adsorvidos na superfície como produtos primários.

CO2 conversion to renewable fuels VI

Palavras-chave: Fotorredução do CO2, combustíveis solares, TiO2, óxido de

grafeno, cobre.

CO2 conversion to renewable fuels VII

Nomenclature

CB Conduction band

CCICED China Council for International Cooperation on

Environment and Development

DRUV Diffuse reflectance UV/Vis spectroscopy

e- Photogenerated electron

EFF Emissions from fossil fuels and cement production

EG Energy gap

IEA International Energy Agency

IFF Carbon intensity of the world economy

IPCC Intergovernmental Panel on Climate Change

FID Flame ionization detector

GC Gas chromatography

GDP Gross domestic product

GHG Greenhouse gas

GO Graphene oxide

GOSAT Greenhouse Gases Observing Satellite

h+ Photogenerated hole

LED Light-emitting diode

MLO Mauna Loa Observatory

NASA National Aeronautics and Space Administration

OCO Orbiting Carbon Observatory

SEM Scanning electron microscopy

TCD Thermal conductor detector

VB Valence band

NHE Normal hydrogen electrode

CO2 conversion to renewable fuels VIII

XPS X-ray photoelectron spectroscopy

CO2 conversion to renewable fuels IX

Table of Contents

AGRADECIMENTOS .................................................................................................... I

ABSTRACT .................................................................................................................. III

RESUMO ......................................................................................................................... V

NOMENCLATURE ................................................................................................... VII

1. INTRODUCTION ................................................................................................... 1

1.1. Overview ............................................................................................................... 1

1.2. Presentation of the Research Unit ...................................................................... 2

1.3. Structure of the thesis ......................................................................................... 3

2. STATE OF THE ART, MOTIVATION AND OBJECTIVES ........................... 5

2.1. Overview of the problematic of CO2 .................................................................. 5

2.2. Photocatalytic conversion of CO2 ....................................................................... 8

2.3. Enhancement of the photocatalytic activity of TiO2 ...................................... 10

2.3.1. Metal Co-catalysts ........................................................................................ 10

2.3.2. Nanostructured carbon materials .................................................................. 11

2.4. Methanol Applications ...................................................................................... 12

2.5. Motivation and objectives of the thesis ............................................................ 17

3. EXPERIMENTAL ................................................................................................ 19

3.1. Chemicals ........................................................................................................... 19

3.2. Synthesis of graphene oxide .............................................................................. 19

3.3. Preparation of GO-TiO2 composites ................................................................ 19

3.4. Preparation of Cu-loaded GOT........................................................................ 20

3.5. Catalysts characterization ................................................................................ 21

3.6. Photocatalytic experiments ............................................................................... 22

4. RESULTS AND DISCUSSION............................................................................ 25

4.1. Characterization of the materials .................................................................... 25

CO2 conversion to renewable fuels X

4.1.1. Thermogravimetric analysis and scanning electron microscopy (SEM) ..... 25

4.1.2. Diffuse reflectance UV/Vis spectroscopy (DRUV) ..................................... 26

4.1.3. X-ray photoelectron spectroscopy (XPS) ..................................................... 27

4.2. Photocatalytic reduction of CO2 ....................................................................... 30

4.2.1. Catalyst screening ........................................................................................ 30

4.2.1.1. Methanol production ................................................................................. 30

4.2.1.2. Ethanol production .................................................................................... 33

4.2.1.3. Acetic acid production. ............................................................................. 35

4.3. Mechanism of reaction ...................................................................................... 36

5. CONCLUSIONS, FUTURE WORK AND FINAL REMARKS ...................... 39

5.1. Conclusions ........................................................................................................ 39

5.2. Future work ....................................................................................................... 40

5.3. Final remarks ..................................................................................................... 40

6. REFERENCES ...................................................................................................... 41

APPENDIX A: CALIBRATION CURVES OF METHANOL, ETHANOL AND

ACETIC ACID ............................................................................................................. 47

List of Tables

Table 1 - Summary of recent studies based on semiconductor materials used in the

photocatalytic conversion of CO2 into methanol. ........................................................... 13

Table 2 - Preparation conditions of photocatalysts. ....................................................... 21

Table 3 - Copper content obtained from XPS analysis. ................................................. 28

Table 4 - Yield of the products formed. ......................................................................... 36

CO2 conversion to renewable fuels XI

List of Figures

Figure 1 - Global CO2 emissions from fossil fuel combustion and cement production

(black dots) and estimating to 2019 (red dots). Reprinted from [1]. ................................ 5

Figure 2 - The CO2 emissions from the top four emitters (China, USA, EU28, India)

and estimating to 2019 (red dots). Reprinted from [1]. .................................................... 7

Figure 3 - Schematic illustration of photoinduced generation of an electron–hole pair in

TiO2 semiconductor that transfers to the surface for CO2 reduction. ............................. 10

Figure 4 - Process of CO2 photoreduction on Cu/TiO2. ................................................ 11

Figure 5 - Schematic diagram of the lab-scale set-up for photoreduction of CO2. ....... 22

Figure 6 - Irradiance spectra of the Heraeus TQ-150 UV-Vis lamp equipped with a

DURAN 50® jacket and transmittance spectrum of the cut-off filter. .......................... 23

Figure 7 - Photocatalytic reactor. .................................................................................. 24

Figure 8 - SEM micrographs for (a) TiO2, (b) GOT, (c) TiO2_Cu(NO3)2, (d)

GOT_Cu(NO3)2, (e) TiO2_Cu2(AC), (f) GOT_Cu2(AC), (g) TiO2_Cu2O, (h)

GOT_Cu2O, (i) EDS spectrum of GOT_Cu(NO3)2. ....................................................... 26

Figure 9 - Diffuse reflectance UV/Vis spectroscopy of some photocatalysts materials.

........................................................................................................................................ 27

Figure 10 - XPS spectra of the Ti 2p and Cu 2p regions for (a and c) TiO2_(CuNO3)2

and (b and d) GOT_(CuNO3)2 composites, respectively. .............................................. 29

Figure 11 - Methanol yield evolution in the experiments performed with different

materials and blank for comparison................................................................................ 31

Figure 12 - Methanol yield at 240 min in the experiments performed with different


Figure 13 - Ethanol yield evolution in the experiments performed with different


Figure 14 - Ethanol yield at 240 min in the experiments performed with different


Figure 15 - Acetic acid yield evolution in the performed experiments. ........................ 35

Figure 16 - Schematic diagram showing mechanism of CO2 photoreduction on Cu-

loaded photocatalyst. Reprinted from [65]. .................................................................... 37

Figure A1 - Calibration curve of methanol. .................................................................. 47

Figure A2 - Calibration curve of ethanol. ...................................................................... 47

Figure A3 - Calibration curve of acetic acid. ................................................................ 48

CO2 conversion to renewable fuels XII

CO2 conversion to renewable fuels 1

1. Introduction

1.1. Overview

As a greenhouse gas, CO2 contributes to global climate change and anthropogenic

emission of CO2 is resulting in an increasing global concern [1]. In this sense, the

development of an efficient process for CO2 conversion is one of the biggest challenges

in environmental research [2]. Considering that solar energy is abundant, clean and a

sustainable energy resource [3], there is much current interest on developing fuels

obtained from sunlight [4].

Photocatalytic reduction of CO2 into fuels, by using water as the reductant and

natural sunlight as the photon source, offers an attractive way to decrease actual CO2

atmospheric concentrations [5]. Previous works already demonstrated that CO2 could be

photocatalytically reduced to hydrocarbons in liquid and gas phase, the efficiency of the

photocatalytic materials for the conversion of CO2, being of critical importance [6, 7].

Solar fuels derived from CO2 can be neutral from the CO2 balance point of view and

they can be also easily transportable as liquid chemicals, such as CH3OH, or even other

chemicals that have been already used in massive quantity (e.g., CH4 and CO) [8].

Titanium dioxide (TiO2) is a classic semiconductor material that has been widely

applied in the field of energy conversion and photocatalysis because of its effectiveness,

low cost, relatively low toxicity and chemical stability [9]. Numerous photocatalytic

applications for TiO2 have been studied, including degradation of pollutants [10], water

photolysis [11], and also CO2 reduction [12]. However, practical application of TiO2

photocatalytic materials is compromised by two inherent limitations, namely the low

quantum yield, which is primarily impaired by the recombination of photo-generated

charge carriers [13], and the poor light-harvesting ability that is restricted by the wide

band gap of TiO2 to the UV spectral range [14].

Graphene and graphene-based materials have stimulated the interest on the design

of high-performance photocatalysts since they can enhance the photoefficiency of the

semiconductors due to the excellent mobility of charge carriers, a large specific surface

area, flexible structure, high transparency and good electrical and thermal conduction

[15].


On the other hand, co-catalysts (such as Cu, Pt, Au, among others) have shown to

increase the photocatalytic conversion of CO2, while disfavoring hydrogen generation

that is one of the main problems during CO2 reduction. It is generally accepted that the

co-catalyst may facilitates the separation of the photo-generated electrons and holes by

trapping electrons, enhancing the photocatalytic activity [16].

In this thesis, GO-TiO2 composites were prepared and applied to the

photocatalytic reduction of CO2 into renewable fuels. The effect of copper metal as co-

catalyst was assessed in the photocatalytic reaction. The main products obtained during

this process were methanol, ethanol and acetic acid.

1.2. Presentation of the Research Unit

The Laboratory of Catalysis and Materials (LCM) in partnership with the

Laboratory of Separation and Reaction Engineering (LSRE) became a national

Associate Laboratory in 2004, in recognition of the capacity of the two units to

cooperate in a stable, competent and effective way in the prosecution of specific

objectives of the National Scientific and Technological Policy. The Associate

Laboratory is based in the Chemical Engineering Department of the Faculty of

Engineering of University of Porto (FEUP), with two external Poles at Instituto

Politécnico de Bragança and Instituto Politécnico de Leiria. FEUP is a public institution

of higher education with financial autonomy and the largest Faculty of the University of

Porto.

The present work is in line with the objectives of the Research Group on Catalysis

and Carbon Materials, namely: development of new catalytic technologies for efficient

energy production and synthesis of high performance carbon-semiconductor

photocatalysts for solar fuels production (methanol from the photoreduction of CO2).

The work was carried out at the LCM laboratories located in the Department of

Chemical Engineering/FEUP (E-301 and E-302A). The most relevant equipment used

in the present work were a lab-scale set-up for the photocatalytic reduction of CO2

equipped with a Heraeus TQ 150 medium pressure mercury vapor lamp, which was

built and optimized in the framework of this MSc Thesis, and a gas chromatograph

system, using a flame ionization detector (FID) and a thermal conductor detector

(TCD).


1.3. Structure of the thesis

The thesis is divided into five chapters:

(i) The first chapter considers the problem that is under investigation

in this work and describes the Research Unit.

(ii) The second chapter presents the state of the art, besides the

motivation and objectives of the thesis.

(iii) The third chapter details the preparation of the materials used in

this study, the description of characterization techniques as well

as the experiments performed.

(iv) The fourth chapter includes the results obtained during the work

performed and the corresponding discussion.

(v) The chapter five resumes the main conclusions resulting from this

work, some suggestions for future work and the final

appreciation.


2. State of the art, motivation and objectives

2.1. Overview of the problematic of CO2

Fossil fuels are currently the most important source of energy due to their

availability, stability, high energy density (e.g., 33 GJ/m3 for gasoline) and high added

value [12]. Nevertheless, the combustion of fossil fuels is accompanied by the

significant drawback of environmental pollution, mainly owing to CO2 emissions and

their associated impacts on the ecosystem [17]. The increase of CO2 emissions is widely

considered as the main driving factor that causes the greenhouse phenomenon leading to

an increase of temperature in the surface of Earth [18].

The rise in greenhouse gas concentrations is believed to result in climate changes

that will have serious environmental, social and economic effects such as: (i) rising sea

levels, (ii) frequency and intensity of extreme weather events, (iii) changes in

agricultural productivity, (iv) extinction of species and (v) number of disease vectors

[18]. In the last years, global CO2 emissions have been growing by ca. 2.5 % yr–1

(Figure 1). CO2 emissions from fossil fuels and cement production (EFF) are estimated

at 37.0 Gt CO2 in 2014 and the perspective is to reach the value of 43.2 Gt CO2, in 2019.

CO2 EFF are generally driven by the gross domestic product (GDP) and the decrease

(improvement) in the carbon intensity of the world economy (IFF) [1].

Figure 1 - Global CO2 emissions from fossil fuel combustion and cement production (black

dots) and estimating to 2019 (red dots). Reprinted from [1].

Besides Europe, there are three countries (China, India, USA) that play a critical

role in emissions growth (Figure 2). Since 2006, China was the world largest emitter of


carbon dioxide, responsible for 28.1% of emissions, followed by USA (14.1%), then

Europe (9.19%), and finally India (6.77%) [1]. The growth of emissions from China

was 4.2% in 2013 [1] and the China Council for International Cooperation on

Environment and Development (CCICED) projected that emissions in China will start

to decrease only by 2050 [19]. China is currently the third largest producer of biofuels

in the world and the Chinese coal sector produces about 40% of the world coal [20].

The USA’s 2.9% growth in emissions in 2013 reversed the previous nation’s trend

of decreasing emissions since 2007 until 2012 as a result of a return to a stronger

economic growth rate (2.2%), and an unusual increase in IFF (0.7%). These results can

be explained because coal has regained some market share from natural gas in the

electric power sector. The decrease of 1.8% of CO2 emissions in Europe in 2013 was a

continued downward trend, due to the relatively low GDP growth rate (0.5%) and IFF

decrease (2.2%). The recent Indian emissions growth was driven by robust economic

growth and by an increase in IFF. This country is known as being the only major

economy with a sustained increase in IFF from 2010 to 2013 [1].

According to the International Energy Agency (IEA), in 2007, the main global

carbon dioxide emissions from fossil fuels combustion are by sector: 41% power

generation, 23% transportation and 17% industry [21].

Carbon footprint is defined as the total amount of carbon dioxide (CO2) produced

directly and indirectly by human activities, usually expressed in equivalent tons of CO2

[22]. This concept motivates companies to develop market products with environmental

attributes, promoting a green economy. Thus, it is expected that the carbon product

labelling makes a decisive pressure on manufacturers and major chains to reduce carbon

emissions from their stores. The government and international institutions have a key

role in defining the protocols and standards of carbon labelling [23].


Figure 2 - The CO2 emissions from the top four emitters (China, USA, EU28, India) and

estimating to 2019 (red dots). Reprinted from [1].

According to the Mauna Loa Observatory (MLO), which has been monitoring

atmospheric CO2 since 1958 in Hawaii, the atmospheric CO2 concentration on Earth

exceeded 400 ppm, for the first time, in May 2013 [24, 25]. Furthermore, the

Intergovernmental Panel on Climate Change (IPCC) released an assessment in 2007,

which recommended to keep atmospheric greenhouse gases below 450 ppm in order to

keep the temperature rise under a crucial 2 ºC level [24].

In order to improve the knowledge about the concentration of atmospheric CO2,

two satellites were designed before 2010 to measure specifically the column-averaged

dry air mole fraction of CO2 (XCO2): (i) GOSAT, the Japanese satellite successfully

launched in January 2009 and (ii) OCO-1, a NASA satellite that got lost because of its

launch failure in February 2009. More recently, in July 2014, NASA has launched a

new satellite, OCO-2, with an instrument very similar to that of OCO-1. Nowadays,

atmospheric CO2 measurement with satellites is one of the most effective approaches

for monitoring the global distributions of greenhouse gases at high spatiotemporal

resolution and carry out a key role in the current study of CO2 [25].


2.2. Photocatalytic conversion of CO2

CO2 reduction into hydrocarbons is a strategy to decrease the CO2 atmospheric

concentrations while producing a new renewable energy source. CO2 reduction can be

achieved by electrocatalysis [3, 26] or photocatalysis [3, 4, 6, 16, 27, 28] and in both

cases water is ideally used as reducing agent, providing the hydrogen atoms/ions

required to react with CO2 to form hydrocarbons. In this way, CO2 is removed from the

atmosphere while hydrocarbons are produced as chemical fuels. In fact, the synthesis of

chemical fuels is one of the most sustainable and practical ways to store energy [5].

The advantage, but also the challenge, of the photocatalytic process is the

possibility to employ natural sunlight as the photon source, offering an attractive and

economic way for the reduction of CO2 to hydrocarbons, in this case known as “solar

fuels” [5]. The concept of solar fuels has been created to denote the strategy based on

the conversion of sunlight into chemicals that could be readily stored and/or transported

[29, 30]. The photocatalytic reduction of CO2 into solar fuels demands a high input

energy to break C=O bond and form C–H bond, involving the participation of multiple

electrons and a corresponding number of protons in the process [16].

In photocatalysis, when a semiconductor is illuminated with photons of energy hν

that is equal to or higher than the semiconductor band-gap EG (hν ≥ EG), these photons

are absorbed and create high energy electron-hole pairs, which dissociate into free

photoelectrons (in the conduction band) and photoholes (in the valence band). The

photo-generated electrons and holes that migrate to the surface of the semiconductor

without recombination can, respectively, reduce and oxidize the reactants adsorbed on

the semiconductor surface [31].

The CO2 photoreduction is a complex series of multiple steps in which several

products may be formed simultaneously [28] (Eqs 1-10) [27]:

hehvTiO2 (1)

Oxidation reaction:

HOhOH 2 ½2 22 (2)


Reduction reactions:

Hydrogen formation:

22 HeH

(3)

CO2 radical formation:

22 COeCO (4)

Formic acid formation:

HHCOeHCO 22 22

(5)

Carbon monoxide formation:

OHCOeHCO 22 22

(6)

Formaldehyde formation:

OHHCHOeHCO 22 44

(7)

Methanol formation:

OHOHCHeHCO 232 66

(8)

Methane formation:

OHCHeHCO 242 288

(9)

Ethanol formation:

OHOHHCeHCO 2522 312122

(10)

Different semiconductors have been applied in the photoreduction of CO2 [4, 16,

27], but TiO2-based materials are the most extensively studied due to of the TiO2

stability, relatively low toxicity and low cost [9]. During the process of CO2

photoreduction with H2O, photo-illumination of the catalyst surface induces the

generation of electron-hole pairs in TiO2. The excited electrons in the conduction band

(CB) of TiO2 could migrate to the surface and reduce CO2 to solar fuels (i.e., CO, CH4,

CH3OH, HCOOH). Meanwhile, the holes left in the valence band (VB) of TiO2 could

oxidize H2O into oxygen (general scheme shown on Figure 3) [4, 16, 27].


Figure 3 - Schematic illustration of photoinduced generation of an electron–hole pair in TiO2

semiconductor that transfers to the surface for CO2 reduction.

The main reasons for the limited efficiency of photocatalytic reduction of CO2

with H2O are: (i) the highly unfavorable one-electron transfer to the formation of CO2

radical (

2CO ) that requires a very negative reduction potential (Eq. 4); (ii) the strong

oxidation power of the photoexcited holes that induce backward reactions, i.e.,

oxidizing the intermediates and products converted from CO2 (iii) the phenomenon of

recombination electrons (e-)/holes (h

+) (Eq. 1), and (iv) hydrogen generation as a

consequence of the presence of water (Eq. 3), that can compete for the electrons in the

conduction band [27].

2.3. Enhancement of the photocatalytic activity of TiO2

As previously mentioned, TiO2 presents some particular drawbacks namely the

low quantum yield [32], which is primarily impaired by the recombination of photo-

generated charge carriers as well as a limited photoactivity in the visible range of

Earth’s solar spectrum. In order to overcome these limitations, several approaches have

been attempted to modify TiO2 such as metal co-catalysts and nanostructured carbon

materials, among others.

2.3.1. Metal Co-catalysts

A metal co-catalyst has the ability to reduce the phenomenon of recombination of

electrons (e-) and holes (h

+) as well as the problem of photocatalytic water reduction

H2O

TiO2

VB

CB CO2

h+

e-

H+ + O2

CH4, CH3OH, etc


(hydrogen generation) [4, 16, 27, 32-36]. By doping metals like copper (Cu), Platinum

(Pt) or Iron (Fe), in the TiO2 lattice, or by coating the TiO2 surface with these metals, a

better performance in the photocatalytic conversion of CO2 can be achieved. These

metals can operate as sinks for the excited TiO2 electrons. Thereby, the metal co-

catalysts promote a decrease of the phenomenon of e– / h

+ pairs recombination as well

as the problem of hydrogen generation [27].

Copper appears as the preferred metal to increase the reduction of CO2, among the

various metal studied. The deposition of copper (I) oxide (Cu2O) on TiO2 not only

reduces the amount of CO2, but also increases the yield of product formation,

particularly methanol [27, 32, 37]. This mechanism is illustrated in Figure 4.

Figure 4 - Process of CO2 photoreduction on Cu/TiO2.

2.3.2. Nanostructured carbon materials

Among different materials that can be selected to prepare composites with TiO2,

carbon materials offer unique advantages such as inert nature and stability, in both acid

and basic media, and the possibility to control their textural and chemical properties.

Recently, nanostructured carbon materials such as carbon nanotubes (CNT), fullerenes

(C60) and graphene have been used to produce carbon-based TiO2 composites [38]. In

particular, graphene oxide (GO) consists of graphene layers decorated with oxygen

functional groups on the graphene basal plane and on the edges, affording abundant and

reactive anchoring sites for the assembly of semiconductor materials [38]. GO can act as


electron acceptor or electron donor, increasing the photocatalytic activity of the

semiconductor towards solar irradiation [16].

2.4. Methanol Applications

Methanol (CH3OH) is a liquid fuel with several applications [39]. For instance, it

can be used in fuel cells [39, 40] as well as a way to store and transport fuels [39, 41].

This high energy density alcohol can be also used as a solvent and anti-freezing agent

and it also presents good efficiency for fuel turbines applied in power generation.

Methanol has also excellent characteristics of storage and transportation [39-42] and can

be used as an alternative fuel for vehicles (mixture of 85% methanol with 15% unleaded

gasoline) [39, 41, 42]. It can be used as a feedstock for biodiesel production or in the

production of formaldehyde, methyl tertiary butyl ether (MTBE), acetic acid, methyl

methacrylate and dimethyl terephthalate. Moreover, methanol can replace oil as a

feedstock for the main products of petrochemical and chemical industry, due to its

ability to be transformed into ethylene and propylene [41].

Methanol demand has increased and it is foreseen that, in the next few years, its

price will rise converting this chemical in a more valuable liquid fuel [39].


Table 1 shows some works related with the production of methanol by the

photocatalytic reduction of CO2. The methanol yield value was achieved by the

following equation (Eq. 11):

(11)

By the analysis of the different works, it was possible to realize that the majority

of photocatalysts with some efficiency for CO2 photoreduction, have a slight

photoresponse under sunlight.

Table 1 - Summary of recent studies based on semiconductor materials used in the

photocatalytic conversion of CO2 into methanol.

Catalyst Methanol yield

(µmol g-1 h-1) Light source Reaction Comments

Year

Ref.

TiO2 0.780

254 nm

8 W Hg lamp

138 µWcm-2

0.15 – 0.6 g catalyst

300 ml of 0.2 M

NaOH solution

6h irradiation

Constant

temperature of 50ºC

TiO2 and Cu/TiO2 were

synthesized by an improved sol-

gel process. Cu/TiO2 and

Cu/P25 were impregnated by

adding CuCl2. Formic acid,

formaldehyde and ethanol were

detected in some catalytic

reactions, in amounts that were

much lower than methanol.

2002

[37]

Degussa P25 6.40

2.0% Cu/P25 10.0

0.6% Cu/TiO2 6.30

1.0% Cu/TiO2 12.0

2.0% Cu/TiO2 20.0

3.3% Cu/TiO2 15.0

6.0% Cu/TiO2 3.30

TiO2 (P25) 135

UV black light

lamps

6 × 10 W

2450 µWcm-2

0.3 g catalyst

300 ml of 1 M

KHCO3 solution.

6h irradiation

Temperature

between

43ºC-100 ºC

Modified catalysts were

prepared by impregnating P25

with a copper nitrate solution.

The methanol yield rises as the

temperature increases

2005

[32]

0.5% CuO/TiO2 197

1.0% CuO/TiO2 265

3.0% CuO/TiO2 442

5.0% CuO/TiO2 213

10% CuO/TiO2 127

3.0% Cu/TiO2 194

3.0% Cu2O/TiO2 224


Table 1 - (Continued)



Year

Ref.

P25 0.176

> 300 nm UV-

Vis

500 W Xe

lamp

0.05 g catalyst

30 ml of 0.08 M

NaHCO3

3h irradiation

Constant

temperature

FeTiO3/TiO2 composites were

synthesized by an hydrothermal

method.

2012

[36]

TiO2 0.175

10% FeTiO3/TiO2 0.338

20% FeTiO3/TiO2 0.462

50% FeTiO3/TiO2 0.298

P25 0.0450 > 400 nm

Visible light

500 W Xe

lamp

TiO2 0.141

10% FeTiO3/TiO2 0.319

20% FeTiO3/TiO2 0.432

50% FeTiO3/TiO2 0.352

InTaO4 62.5* 390 - 770 nm

100 mW

Xe lamp

0.1g catalyst

50 mL of desionized

water

2 h irradiation

Constant


Ni@NiO/InTaO4-N

photocatalyst was obtained by

loading 3.2 wt % Ni on InTaO4-

N by impregnation of an

aqueous Ni(NO3)2 solution.

2011

[43]

InTaO4-N 137*

Ni@NiO/InTaO4-N 175*

TiO2 2.16

254 nm

UV lamp

10 W 0.4 g catalyst

400 ml of 0.2 M

NaOH and 0.2 M

Na2SO3

8 h irradiation

Constant


N-Ni/TiO2 were prerared by an

improved sol-gel method.

2011

[44]

N/TiO2 30.7

Ni/TiO2 26.8

N-Ni/TiO2 60.3

TiO2 0.450

365 nm UV

lamp 10 W

N/TiO2 21.0

Ni/TiO2 7.50

N-Ni/TiO2 31.7

TiO2 0.130 400 - 780 nm

Incandescent

lamp

N/TiO2 7.58

Ni/TiO2 3.59

N-Ni/TiO2 15.1

CdS 40.2

> 400 nm

UV light 500

W Xe lamp

0.2 g catalyst

0.8 g of sodium

hydroxide and 2.52

g of absolute sodium

sulfite were

dissolved in 200

distilled water

5 h irradiation

The photocatalysts were

prepared the hydrothermal

method using the corresponding

salt and thiourea in an ammonia

bath.

2011

[45] Bi2S3 62.8

Bi2S3/CdS 123

Bi2WO6 SSR 0.640 Visible light

irradiation 2 h irradiation

An anion exchange strategy is

explored to synthesize Bi2WO6

hollow microspheres that show

a high CO2 adsorption capacity.

2012

[46]

Bi2WO6 HMSs 16.3





Year

Ref.

ZnS deposited on

MMT

1.41*

254 nm

8 W Hg lamp

ZnS-MMT catalyst

(1 g l−1

) was suspended

in the 0.2 M NaOH

solution

24 h irradiation

Two stirred batch annular

reactors with three quartz glass

tubes of different diameters

(3.5, 4.0 and 4.5 cm).

It was found that the formation

of the products depends on the

volume of liquid phase and the

reactor diameter.

2011

[47]

TiO2 14.4

400 nm

visible light

irradiation

500 W Xe

lamp

and

254 nm

UV irradiation

16 W Hg lamp

0.4 g catalysts

400 ml distilled

water

3 h irradiation

Temperature: 40ºC

Ag/TiO2 nanocomposites were

obtained by microwave assisted

chemical reduction method.

2014

[35]

0.5% Ag/TiO2 60.0*

1.0% Ag/TiO2 91.7*

1.5% Ag/TiO2 100*

2.0% Ag/TiO2 125*

2.5% Ag/TiO2 135

3.0% Ag/TiO2 130*

3.5% Ag/TiO2 113*

4.0% Ag/TiO2 108*

TiO2 0

Visible

irradiation 2.5% Ag/TiO2 29.9

TiO2 13.6

UV irradiation

2.5% Ag/TiO2 43.4

1.0% NiO-InTaO4 1.39

Visible light

irradiation

500 W

halogen lamp

0.14 g catalyst

50 ml of 0.2 M

KHCO3 solution

20 h irradiation

Room temperature

NiO was prepared by

impregnation and then by

reduction-oxidation.

2007

[48]

KATI66

0.190

254 nm

UV 8 W Hg

lamp

0.1 g catalyst

100 ml of 0.2 M

NaOH

24 h irradiation

KATI66 (kaolinite/TiO2)

composite was prepared using

thermal hydrolysis of

kaolinite/titanyl sulphate

suspension.

2011

[2]

DegussaP25 0.0300*

2%Cu/TiO2

20.0

254 nm

UV Hg lamp

0.3 g catalyst

300 ml 0.2 M NaOH

solution

30 h irradiation

The temperature was

monitored

continuously

Cu/TiO2 was synthesized with a

sol-gel process.

The precursor CuCl2 performed

better than the copper acetate.

2004

[49]

2%Cu/TiO2 0.330 365nm

UVA



Catalyst Methanol Yield


Year

Ref.

7.0% AgBr/TiO2 8.60* Visible light

irradiation

>420 nm

150 W Xe

lamp

0.5 g catalyst

100 ml of 0.2 M

KHCO3 solution

5 h irradiation

Room temperature

AgBr/TiO2 was prepared by the

deposition-precipitation method.

2011

[50]

11.6% AgBr/TiO2 13.6*

23.2% AgBr/TiO2 15.6

46.4% AgBr/TiO2 11.6*

175 % AgBr/TiO2 8.10*

TiO2 0.0300*

254 nm

UV 8 W Hg

lamp

1g l-1

catalyst

24 h irradiation

TiO2 and Ag/TiO2 catalysts

were prepared by the sol–gel

process.

2010

[34]

1% Ag/TiO2 0.0400*

3% Ag/TiO2 0.0400*

5% Ag/TiO2 0.0500*

5% Ag/TiO2 0.0800*

1.2% Cu/TiO2 0.460

365 nm

Hg lamp

16 W cm-2

75ºC constant

temperature

Methanol yield was studied with

the copper weight change and

also the light intensity (1-16 W

cm-2

).

2005

[51]

TiO2/N-100 10.0 UV-Vis lamp

300-600 nm

0.6 g catalyst

800 ml of water

2 h irradiation

65 ºC

TiO2/N-100 was synthesized by

hydrothermal reaction of

amorphous anatase TiO2 and

NH4OH at 100 ºC.

2014

[52]

TiO2 17.7

254 nm

UV-lamp

0.97 mW cm-2

0.5 g catalyst

500 ml of 0.1 M

NaHCO3

12 h irradiation

25ºC constant

temperature

Cu or Co incorporated

TiO2/ZSm-5 were prepared by a

sol-gel method.

2015

[53]

TiO2/ZSM5 26.5

Cu-TiO2/ZSM5 50.1

Co-TiO2/ZSM5 35.1

TiO2 0.0330

Visible light

irradiation

>400 nm

500 W Xe

lamp

0.05 g catalyst

50 ml distilled water

containing 0.08M

NaHCO3, 0 and 0.08

M HCl

9 h irradiation

Constant

temperature

Mesoporous graphene and

tourmaline single and co-doped

TiO2 composites were prepared

by the sol–gel method.

The yield of methanol was

studied by varying the amount

of catalyst (0.5 to 2.5 g L-1

) of

0:08 M NaHCO3.

G (Graphene) serves as the

transporter of photogenerated

electrons from anatase TiO2.

T (Tourmaline) serves as the

acceptor of photogenerated

electrons.

2014

[54]

1%G/TiO2 0.120

1%T/TiO2 0.240

1%G/1%T/TiO2 0.390

1%G/1.5%T/TiO2 0.450

1%G/2%T/TiO2 0.620

1%G/2.5%T/TiO2 0.720

1%G/3%T/TiO2 0.680

* these values have a small inaccuracy error because they were removed from the graph’s article since it

doesn’t show the numerical values of methanol yield.


2.5. Motivation and objectives of the thesis

The Master in Environmental Engineering is a multidisciplinary course that

provides an integrated approach in order to develop scientific and technological

solutions to existing environmental problems, while promoting sustainable

development.

The CO2 photoreduction into renewable fuels is seen as a promising solution to

reduce the atmospheric concentrations of CO2 and a potential way of creating added-

value chemical products. The development of a selective and high yield photocatalytic

process, which uses CO2 as a feedstock to produce hydrocarbons has a growing

environmental importance. This work has the aim of developing different photocatalysts

and to study their performance in CO2 photoconversion into renewable fuels. In this

sense, the following specific objectives were defined:

Synthesis of TiO2 and graphene oxide-TiO2 (GOT) composites;

Synthesis of Cu loaded TiO2 and GOT catalysts;

Characterization of the prepared materials using different techniques;

To study the performance of all catalysts in the photoreduction of CO2 under

UV/Vis irradiation, aiming catalysts with high activity and stability;

To evaluate the performance of the photocatalysts, with the determination

and quantification of the main subproducts obtained during the process.


3. Experimental

3.1. Chemicals

Ammonium hexaflurorotitanate(IV) ((NH4)2TiF6, > 99.99%), boric acid (H3BO3,

> 99%), ammonium persulfate ((NH4)2S2O8, > 98%), sulphuric acid (H2SO4, > 95%),

copper(II) nitrate trihydrate (Cu(NO3)2 3H2O, > 98%) and copper(II) acetate

(Cu(CH3COO)2, 98%) were obtained from Sigma-Aldrich. Copper(I) oxide (Cu2O,

97%) and copper(II) chloride (CuCl2) were purchased from Riedel-de-Haën. Sodium

hydroxide (NaOH, 98 wt.%) was obtained from Panreac.

3.2. Synthesis of graphene oxide

GO was synthesized from graphite (particle size ≤ 20 µm, from Sigma-Aldrich)

by a modified Hummers method as described elsewhere [55, 56]. In a typical procedure,

50 mL of H2SO4 was added gradually with stirring and cooling to a 500 mL flask

containing 2 g of graphite. Then 6 g of potassium permanganate (KMnO4) was added

slowly to the mixture. The suspension was continuously stirred for 2 h at 35 ºC. After

that, it was cooled in an ice bath and subsequently diluted by 350 mL of deionized

water. Then H2O2 (30% w/v) was added in order to reduce residual permanganate to

soluble manganese ions, appearing a bright yellow color in the suspension. The

oxidized material was purified with a 10% HCl solution and then the suspension was

filtered, washed several times with water until achieve a neutral pH in the resulting

water, and dried at 60 ºC for 24 h to obtain graphite oxide. The resulting material was

dispersed in a given volume of water and sonicated in an ultrasound bath (ultrasonic

processor UP400S, 24 kHz) for 1 h. The sonicated dispersion was centrifuged for 20

min at 3000 rpm to remove unexfoliated graphite oxide particles from the supernatant

and the obtained suspension of graphene oxide was then used for the synthesis of GOT

composites.

3.3. Preparation of GO-TiO2 composites

GO-TiO2 (referred as GOT) was synthesized by the liquid phase deposition

(LPD) method at room temperature, as described elsewhere [57]. Ammonium

hexafluorotitanate (IV), (NH4)2TiF6 (0.1 mol/L), and boric acid, H3BO3 (0.3 mol/L),


were added to a certain amount of the GO dispersion heated at 60 ºC for 2 h under

vigorous stirring. The material was separated by filtration, washed with water and dried

at 100 ºC under vacuum for 2 h. The post-treatment under N2 atmosphere at 200 ºC was

established in previous experiments, taking into account the crystallinity of TiO2

particles and the stability of the GO material at that temperature. The carbon loading (4

wt.%) was selected taking into account the highest photocatalytic activity obtained with

this GOT composite in previous works [57, 58]. During the thermal treatment, anatase

TiO2 particles were exclusively formed at such temperature. Bare TiO2 was prepared

using the same methodology but without the addition of GO (referred as TiO2). The

photocatalyst from Evonik Degussa Corporation (P25) was also used as reference

material.

3.4. Preparation of Cu-loaded GOT

The photocatalysts were prepared by adapting the procedure described by Wu et

al. for Cu-loaded TiO2 [51]. The copper precursor was directly added during the

preparation method of the GOT composite.

Different copper precursors, either copper chloride (CuCl2), copper acetate

(Cu2(Ac)), copper nitrate (Cu(NO3)2) or copper (I) oxide (Cu2O) were used. The amount

of copper precursor was adjusted to obtain the desired Cu loading of ca. 2 wt.%. The

resulting material was separated by filtration, washed with water and dried at 100 ºC.

Then, the catalysts were treated in a furnace at 150 ºC for 3 h by 3 ºC min-1

from room

temperature, then raised 2 ºC min-1

to 500 ºC and stayed for 5 h under air atmosphere.

Cu (I) specie is predicted to be formed during this calcination step [51].

The TiO2-Cu2O composites were prepared following the same procedure but

without the addition of GO. Table 2 summarizes the preparation conditions and the

label used for the prepared catalysts.


Table 2 - Preparation conditions of photocatalysts.

Photocatalyst Cu (wt.%) Copper precursor Post-treatment

P25

TiO2

GOT

--

--

--

--

--

--

--

200 ºC (N2)

200 ºC (N2)

TiO2_CuCl2 ~ 2.0 CuCl2 150 ºC ( 3h, air) +

500 ºC (5h, air)

TiO2_Cu2(Ac) ~ 2.0 Cu2(OAc)4 150 ºC ( 3h, air) +

500 ºC (5h, air)

TiO2_Cu(NO3)2 ~ 2.0 Cu(NO3)2 150 ºC ( 3h, air) +

500 ºC (5h, air)

TiO2_Cu2O ~ 2.0 Cu2O --

GOT_CuCl2 ~ 2.0 CuCl2 150 ºC ( 3h, air) +

500 ºC (5h, air)

GOT_ Cu2(Ac) ~ 2.0 Cu2(OAc)4 150 ºC ( 3h, air) +

500 ºC (5h, air)

GOT_Cu(NO3)2 ~ 2.0 Cu(NO3)2 150 ºC ( 3h, air) +

500 ºC (5h, air)

GOT_Cu2O ~ 2.0 Cu2O --

3.5. Catalysts characterization

Thermogravimetric (TG) analysis of the composites were performed using a STA

490 PC/4/H Luxx Netzsch thermal analyser, by heating the sample in N2 flow from 50

ºC to 1000 ºC at 20 ºC min-1

.

The morphology of the materials was studied by scanning electron microscopy

(SEM) using a FEI Quanta 400FEG ESEM/EDAX Genesis X4M microscope.

The optical properties of the samples were analyzed by diffuse reflectance UV/Vis

spectroscopy using a JASCO V-560 UV/Vis spectrophotometer, equipped with an

integrating sphere attachment (JASCO ISV-469) and barium sulfate was used as a

reference. The reflectance spectra were converted by the instrument software (JASCO)

to equivalent absorption Kubelka-Munk units.

X-ray photoelectron spectroscopy (XPS) was performed in a Kratos AXIS Ultra

HSA using a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15 kV (90

W), in FAT mode (Fixed Analyser Transmission), with a pass energy of 40 eV for

regions of interest and 80 eV for survey.


3.6. Photocatalytic experiments

The photocatalytic runs were carried out in a cylindrical glass immersion photo-

reactor filled with 250 mL of 0.2 M NaOH solution and containing 250 mg of catalyst

(catalyst load of 1 g L−1

). A schematic diagram of the photocatalytic reactor is shown in

Figure 5.

Figure 5 - Schematic diagram of the lab-scale set-up for photoreduction of CO2.

A Heraeus TQ 150 medium-pressure mercury vapor lamp (exc = 254, 313, 365,

436 and 546 nm) was located axially in the reactor and held in a quartz immersion tube.

A DURAN glass cooling jacket was used for irradiation in the near-UV to visible light

H2O in

H2O out

Valve

Liquid Sample

Extracted Sample

Stirrer Bar

Photo ReactorFiltered Samples

Hg Lamp

CO2 + He

NaOH 0.2M + Catalyst

CO2 He

MFC MFC

Vent

GC-FID HPLC-UV

GC-TCD

Gas Sample

Vent

Power Supply


range (exc > 350 nm) - Figure 6. During the photocatalytic experiments, the temperature

was kept at around 25 ºC. A photograph of the experimental set-up is shown in Figure 7.

Prior to the light irradiation, the above system was degased by using a He and

CO2 flow overnight (10 cm3 min

−1 and 2 cm

3 min

−1, respectively) to saturate the

solution. The initial pH value of 0.2 M NaOH solution was approximately pH 13 and

the pH value of the CO2-saturated NaOH solution was nearly pH 7. Then, the reactor

was irradiated during 360 min. To maintain CO2 saturation during measurements, He

and CO2 flow rates were maintained at 27 cm3 min

−1 and 3 cm

3 min

−1, respectively.

Figure 6 - Irradiance spectra of the Heraeus TQ-150 UV-Vis lamp equipped with a DURAN

50® jacket and transmittance spectrum of the cut-off filter.

The gas product was analyzed by an online gas chromatograph (GC) equipped

with a thermal conductivity detector (TCD), using a capillary column (Carboxen 1010

Plot. Supelco).

In order to determine the formation of methanol and ethanol, liquid samples were

periodically withdrawn and analysed by gas chromatography (DANI GC-1000) using a

capillary column (WCOT Fused Silica 30 m, 0.32 mm i.d., coated with CP-Sil 8 CB

low bleed/MS 1 m film) and a flame ionization detector (FID). All the photocatalytic

experiments were replicated two times. The reported data corresponds to the average

over two independent runs.


Concentrations of some organic acids resulting from the CO2 photo degradation

were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a UV

detector. An Alltech OA-1000 column (300 mm × 6.5 mm) was used as stationary

phase, working at room temperature, under isocratic elution with a solution of H2SO4 5

mM at a flow rate of 0.5 mL min−1

and using an injection volume of 15 μL.

Figure 7 - Photocatalytic reactor.


4. Results and discussion

4.1. Characterization of the materials

4.1.1. Thermogravimetric analysis and scanning electron microscopy (SEM)

The copper content (wt.%) in the composite materials was determined by

thermogravimetric analysis (not shown); both GOT and the Cu-loaded composites were

submitted to a thermal treatment under N2 and the weight loss was monitored. The

copper content in the composite corresponds to the difference between the weight loss

observed for the Cu-loaded composite and that of GOT composite. The obtained results

are in a good agreement with the expected Cu content (c.a. 2 wt.%).

The representative SEM images of TiO2, GOT composite as well as the copper-

loaded photocatalysts are shown in Figures 8a-h. The morphology of TiO2 consists of

spherical particles that aggregate originating clusters of TiO2 particles (Figure 8a). The

GOT composite consists of TiO2 particles aggregated on the GO layers, forming GOT

platelets (Figure 8b) with a well TiO2 distribution on both sides of the graphene oxide

nanosheets [57].

The surface morphology of the Cu-loaded TiO2 photocatalysts prepared with

Cu2(OAc)4 and Cu2O (Figure 8e and 8g, respectively) seems similar to that of bare TiO2

(Figure 8a). On the contrary, the morphology observed for TiO2_Cu(NO3)2 (Figure 8c)

was different than that obtained for bare TiO2, presenting larger clusters. Regarding the

Cu-loaded GOT composites (Figures 8d, 8f and 8h), the morphologies obtained were

slightly different than that observed for GOT (Figure 8b), since the platelets were not so

notorious when Cu was loaded. All the materials presented a quite homogeneous

morphology, but the better assembling of TiO2 nanoparticles and GO platelets seems to

be achieved with GOT.

The EDS spectra obtained for the composites (as shown in Figure 8i for

GOT_Cu(NO3)2), revealed Ti, O and C peaks, that were associated to TiO2 and to GO

(the O peak resulting from TiO2 and from the oxygenated surface groups present in the

chemical structure of GO). It is difficult to infer about the presence of Cu, probably due

to the low amounts loaded, even so there is some indication of the Cu presence in the

EDS spectrum (Figure 8i).


4.1.2. Diffuse reflectance UV/Vis spectroscopy (DRUV)

The optical properties of selected samples were analyzed using the diffuse

reflectance DRUV-Vis spectra expressed in terms of Kulbelka-Munk equivalent

absorption units, as shown in Figure 9 for P25, GOT, TiO2_Cu(NO3)2, GOT_Cu(NO3)2,

TiO2_Cu2O, GOT_Cu2O and TiO2_CuCl2.

For P25, the characteristic absorption edge rising at 330 nm is associated with

the excitation of the O 2p electron in the valence band (VB) to the Ti 3d state in the

conduction band (CB) [59]. Regarding the Cu-loaded TiO2 catalysts, the band edge is

slightly expanded into both UV and visible regions (Figure 9). This effect could be due

i) GOT_Cu(NO3)2

c) TiO2_Cu(NO3)2

d) GOT_Cu(NO3)2 e) TiO2_Cu2(AC) f) GOT_Cu2(AC)

g) TiO2_Cu2O h) GOT_Cu2O

b) GOT a) TiO2

Figure 8 - SEM micrographs for (a) TiO2, (b) GOT, (c) TiO2_Cu(NO3)2, (d)

GOT_Cu(NO3)2, (e) TiO2_Cu2(AC), (f) GOT_Cu2(AC), (g) TiO2_Cu2O, (h) GOT_Cu2O, (i)

EDS spectrum of GOT_Cu(NO3)2.


to the additional energy levels created by the copper ions in the band gap of TiO2 [49,

60, 61].

Figure 9 - Diffuse reflectance UV/Vis spectroscopy of some photocatalysts materials.

As expected, for GOT and Cu-loaded GOT composites, a higher absorption in the

visible spectral range is clearly observed, in comparison with GO-free materials, i.e.,

P25, TiO2_Cu2O, TiO2_Cu(NO3)2 and TiO2_CuCl2, due to the introduction of the GO

material (Figure 9). This increase in the absorption promoted by GO has been ascribed

to both the capacity of the carbon phase to absorb light and also to the creation of an

electronic inter phase interaction between GO and TiO2 phase, as observed for other

carbonaceous materials combined with TiO2 [38, 62].

4.1.3. X-ray photoelectron spectroscopy (XPS)

XPS measurements were carried out over the photocatalysts in order to determine

the oxidation state of the copper species existing in the photocatalysts. As example,

figures 10a and 10b show the XPS spectra of the Ti 2p region for TiO2_Cu(NO3)2 and

GOT_Cu(NO3)2 composites, respectively. From the Ti 2p region, two peaks were

detected and centered at 459.1 and 464.8 eV, corresponding to Ti 2p3/2 and Ti 2p1/2,

0

1

2

3

4

250 300 350 400 450 500 550 600

Ku

belk

a M

un

k (

u.a

.)

λ (nm)

P25

TiO₂_Cu₂O

TiO₂_Cu(NO₃)2

TiO₂_CuCl2

GOT_Cu(NO₃)2

GOT

GOT_Cu₂O


respectively. In addition, the splitting between both peaks was found at 5.7 eV. These

results indicate a Ti4+

chemical state, typical of TiO2 as previously reported [57, 60].

Table 3 presents the amounts of copper introduced on the surface of the materials

calculated from the XPS data. The photocatalysts prepared with Cu(NO3)2 as Cu

precursor present the largest amount of Cu in the surface of these materials (0.7% and

0.6% for TiO2_Cu(NO3)2 and GOT_Cu(NO3)2, respectively). The low amounts of Cu

detected (in comparison with the theoretical 2 wt.%) indicate that copper was dispersed

mostly inside the structure of the prepared catalysts, rather than in the catalyst surface.

The XPS Cu 2p spectra of the TiO2_Cu(NO3)2 and GOT_Cu(NO3)2 samples are

shown in Figures 10c and 10d, respectively. Cu 2p3/2 characteristic peaks for metallic

Cu, Cu2O and CuO appear at 932.0, 932.7 and 933.8 eV [63]. The Cu 2p3/2 and Cu 2p1/2

binding energies of the prepared materials were found to be respectively ca. 932.4 and

952.4 eV, confirming the presence of Cu(I). However the binding energies for CuO

were 1 eV above those for Cu2O which are 933.8 and 953.8, respectively [61]. It has

been reported that, according to the position and the shape of the peaks, the copper on

the surface of the catalyst may exist in multiple-oxidation states but Cu(I) is the primary

species [49]. These results are in agreement with previous works, where lower oxidation

state of Cu was observed at higher calcination temperatures [51, 60].

Table 3 - Copper content obtained from XPS analysis.

Photocatalyst CuXPS (%)

TiO2_CuCl2 ~ 0.5

TiO2_Cu2(Ac) ~ 0.3

TiO2_Cu(NO3)2 ~ 0.7

GOT_CuCl2 ~ 0.5

GOT_ Cu2(Ac) ~ 0.2

GOT_Cu(NO3)2 ~ 0.6


Figure 10 - XPS spectra of the Ti 2p and Cu 2p regions for (a and c) TiO2_(CuNO3)2 and (b

and d) GOT_(CuNO3)2 composites, respectively.

454459464469

Inte

nsi

ty (

a.u

)

Binding Energy (eV)

Ti 2p

TiO₂_Cu(NO3)2

5.7 eV

454459464469

Inte

nsi

ty (

a.u

)

Binding Energy (eV)

Ti 2p

5.7 eV

GOT_Cu(NO3)2

928933938943948953958

Inte

nsi

ty (

a.u

)

Binding Energy (eV)

Cu 2p

TiO₂_Cu(NO3)2

20 eV

928933938943948953958

Inte

nsi

ty (

a.u

)

Binding Energy (eV)

Cu 2pGOT_Cu(NO3)2

20 eV

2p1/2 2p3/2

2p1/2 2p3/2

2p1/2

2p3/2

2p1/2

2p3/2

a)

b)

c)

d)


4.2. Photocatalytic reduction of CO2

4.2.1. Catalyst screening

4.2.1.1. Methanol production

As previously indicated, water can act as the reducing agent in the photocatalytic

reduction of CO2. However, a NaOH aqueous solution is used normally in this reaction

due to the following reasons: (i) the amount of dissolved CO2 is higher because NaOH

solution dissolves more CO2 than pure water and (ii) the recombination of electron-

holes pairs is lower because the highly concentrated OH ions in aqueous solution could

act as strong holes scavengers favoring generation of •OH radicals. The most common

concentration of NaOH reported is 0.2 M [4, 49, 64, 65] and this concentration was

adopted in the present work.

The methanol yield evolution during the photocatalytic experiments for the ten

synthesized heterogeneous catalysts as well as for the commercial TiO2 (P25) is

depicted in Figure 11 and 12, while the results for ethanol are shown in Figures 13 and

14. Photocatalytic experiments were typically carried out during 6 h under near-

UV/Visible irradiation.

Blank experiments were also conducted to confirm that the hydrocarbon

production was due to the photoreduction of CO2. The first blank was UV/Vis light

irradiation without photocatalyst, the second blank was performed in dark conditions

with both photocatalyst and CO2 under the same experimental conditions (not shown),

and the third one was over the illuminated photocatalyst in the absence of CO2 (not

shown). Since the results show that no hydrocarbon production was detected in the

above blank tests, as previously reported [35, 44, 52, 66, 67], only the results obtained

with the first blank are shown in Figures 11 and 13, for methanol and ethanol

production, respectively).

In general, the photocatalysts were efficient for methanol generation with the

exception of bare TiO2, GOT_CuCl2 and the catalysts prepared using Cu2O as copper

precursor (i.e., TiO2_Cu2O and GOT_Cu2O, respectively).


Figure 11 - Methanol yield evolution in the experiments performed with different materials and

blank for comparison.

The highest methanol production was observed at 240 min of UV-Vis light

irradiation and the best four catalysts were identified as follows: GOT > TiO2-Cu(NO3)2

GOT-Cu(NO3)2 > GOT-Cu2(Ac) with a maximum methanol yield of 163, 153, 146

and 138 µmol g-1

, respectively (Figure 12). The efficiency of the photocatalytic process

depends on the copper precursor used. Among the Cu-loaded composites,

TiO2_Cu(NO3)2 and GOT_Cu(NO3)2 exhibited the highest photocatalytic activity under

near-UV/Vis, i.e., these prepared with the Cu(NO3)2 precursor. This photocatalytic

activity may be related with the largest amount of Cu on the surface, in comparison to

the other Cu-loaded photocatalysts (derived from the XPS analysis, showed in Table 3).

However, among all the photocatalysts, the highest methanol formation was found

for the GOT composite under near-UV/Vis irradiation (exceeding that of P25 and bare

TiO2 photocatalysts). The high performance of GOT was attributed to the good

assembly and interfacial coupling between the GO sheets and TiO2 nanoparticles (as can

be observed in the SEM micrograph of this composite, Figure 8b), acting as efficient

electron acceptor and donor [38, 57]. Recently, Tan et al. [68] have reported the

photocatalytic reduction of CO2 over reduced graphene oxide (rGO)-TiO2 hybrid

nanocrystals synthetized though a solvothermal route. The reaction was carried out

under gas condition using water vapour as reductant. Under these conditions, CH4 was

obtained as main by-product of reaction. In that work the authors indicate that the

0

20

40

60

80

100

120

140

160

180

0 60 120 180 240 300 360

Me

tha

no

l yie

ld (

µm

ol g

-1)

Time (min)

P25

TiO₂

GOT

TiO₂_CuCl₂

TiO₂_Cu₂(Ac)

TiO₂_Cu(NO₃)₂

TiO₂_Cu₂O

GOT_CuCl₂

GOT_Cu₂(Ac)

GOT_Cu(NO₃)₂

GOT_Cu₂O

blank


enhancement of the photocatalytic activity was attributed to the close contact between

TiO2 and rGO that can accelerate the transfer of photogenerated electrons on TiO2 to

rGO preventing efficiently charge recombination.

Figure 12 - Methanol yield at 240 min in the experiments performed with different materials

and blank for comparison

The methanol production with GOT strongly increased in the first 60 min of

reaction, after that, the increase was less pronounced, reaching the maximum at 240 min

of UV/Vis irradiation (Figure 11). It is also worth noting that the presence of copper in

Cu-TiO2 composites, that were not prepared with Cu2O, promotes a strong increase in

the efficiency of the photoreduction process when compared with bare TiO2 (Figure 12).

As previously mentioned, copper can act as an electron trapper and prohibits the

recombination of electrons and holes, increasing the photoefficiency. However, for the

GOT material, the presence of co-catalyst decreased the photoactivity in all the

composites. In general, the photocatalytic activity of the Cu-loaded GOT composites

seems to be related with the differences in the morphology derived from the SEM

analysis (Figure 8), indicating that the variation in the self-assembling of GO with TiO2

particles is crucial for their photocatalytic performance. For future work, is here

recommended to impregnate Cu over GOT rather than mixing the GO, TiO2 and Cu

precursor together.

P-2

5

TiO₂

GO

T

TiO₂_

Cu

Cl₂

TiO₂_

Cu₂(

Ac

)

TiO₂_

Cu

(NO₃)₂

TiO₂_

Cu₂O

GO

T_

Cu

Cl₂

GO

T_

Cu₂(

Ac

)

GO

T_

Cu

(NO₃)₂

GO

T_

Cu₂O

0

50

100

150

Meth

an

ol yie

ld (

µm

ol g

-1)

240 m

in

Catalysts


It is also noticed that during the photocatalytic reaction, a slight decrease in the

CO2 formation was observed for GOT composite after 240 min of irradiation whereas

the photocatalytic activity for the Cu-loaded photocatalysts was maintained. In this

sense, the photocatalyst seems to deactivate after a long period of irradiation. Three

possible reasons are: (i) the adsorption or accumulation of intermediate products on the

semiconductor surface that could occupy the photocatalytic reaction centers and also

hinder the adsorption of CO2 or H2O; (ii) the adsorption of hydrocarbon fuel products

could affect the adsorption of the reactants, and (iii) the contamination of the surface

photocatalyst prevents the absorption of light and at the same time to leads less

electron/hole pairs.

4.2.1.2. Ethanol production

Regarding the ethanol production, Figure 13 shows the results obtained with all

materials tested. The production of ethanol under non-catalytic conditions (blank) was

negligible.

For the photocatalysts TiO2, GOT_CuCl2 and those prepared using Cu2O as

copper precursor (i.e., TiO2_Cu2O and GOT_Cu2O, respectively) any photocatalytic

activity was observed, as in the case of methanol.

Figure 13 - Ethanol yield evolution in the experiments performed with different materials and

blank for comparison.

0

200

400

600

800

1000

0 60 120 180 240 300 360

Eth

an

ol yie

ld (

µm

ol g

-1)

Time (min)

P25

TiO₂

GOT

TiO₂_CuCl₂

TiO₂_Cu₂(Ac)

TiO₂_Cu(NO₃)₂

TiO₂_Cu₂O

GOT_CuCl₂

GOT_Cu₂(Ac)

GOT_Cu(NO₃)₂

GOT_Cu₂O

blank


The highest ethanol yield was found for TiO2_Cu(NO3)2, followed by the GOT

composite (872 and 479 µmol g-1

of catalyst, respectively at 240 min of irradiation,

Figure 14). Thus, these two catalysts performed better than all the other tested materials

regarding methanol and ethanol products. On the other hand, no ethanol formation was

observed when the reaction was carried out with P25, indicating that, in general, the

presence of GO and copper as co-catalysts lead to composites with improve

photoefficiency compared to both bare TiO2 and P25.

The main hydrocarbons detected during the reaction were both methanol and

ethanol, although the presence of other hydrocarbons such as propanol has been also

identified. These results are in agreement with literature where alcohols, such as

methanol and ethanol, were the major products of CO2 photoreduction in aqueous

solution [27].

Figure 14 - Ethanol yield at 240 min in the experiments performed with different materials and

blank for comparison

It has been also noticed that among all the prepared photocatalysts, the largest

activity for ethanol production was obtained with the catalysts prepared with Cu(NO3)2

as copper precursor, for both ethanol and methanol production. The lowest

photocatalytic activity was obtained with CuCl2 and Cu2O as copper precursors and

could be justified by the amount of the organic residues (e.g., carbon or hydrocarbons)

P-2

5

TiO₂

GO

T

TiO₂_

Cu

Cl₂

TiO₂_

Cu₂(

Ac

)

TiO₂_

Cu

(NO₃)₂

TiO₂_

Cu₂O

GO

T_

Cu

Cl₂

GO

T_

Cu₂(

Ac

)

GO

T_

Cu

(NO₃)₂

GO

T_

Cu₂O

0

200

400

600

800

1000

Eth

an

ol yie

ld (

µm

ol g

-1)

240 m

in

Catalysts


or inorganic ions (e.g., Cl–) that may still be present on the catalyst surface even after

calcination. These surface contaminants could react with CO2 and H2O under light

irradiation, interfering with the photoinduced reactions and influencing the

photocatalytic activity [27]. Therefore, regarding future work, Cu(NO3)2 is

recommended as precursor over a GOT material prepared separately.

4.2.1.3. Acetic acid production.

Figure 15 shows the photocatalytic formation of acetic acid (undesired product)

by the tested materials. The formation of other organic acids such as oxalic acid, lactic

acid and formic acid have been also observed, but in lower quantities. In general, all the

tested catalysts produced a considerable amount of acetic acid. These observations are

in agreement with previous results reported for Cu-TiO2 [16] where acetic acid was the

main by-product of the reaction [69].

Figure 15 - Acetic acid yield evolution in the performed experiments.

Table 4 summarizes the yields of all the byproducts determined (methanol,

ethanol and acetic acid, µmol g-1

h-1

) after 360 min of irradiation. At the end of the

reaction, TiO2_Cu(NO3)2 was the best material for the production of both methanol and

ethanol production (23.9 and 124 µmol g-1

h-1

, respectively), and the acetic acid yield

was negligible (0.870 µmol g-1

h-1

). The largest amount of undesired acetic acid was

0

20

40

60

80

0 60 120 180 240 300 360

Ac

eti

c A

cid

yie

ld (

µm

ol g

-1)

Time (min)

P25

GOT

TiO₂_CuCl₂

TiO₂_Cu₂(Ac)

TiO₂_Cu(NO₃)₂

GOT_CuCl₂

GOT_Cu₂(Ac)

GOT_Cu(NO₃)₂


produced by P25 (8.71 µmol g-1

h-1

) probably due to the high catalytic activity and low

selectivity of this material in photocatalytic reactions.

Table 4 - Yield of the products formed.


(µmol g-1

h-1

)

Ethanol yield

(µmol g-1

h-1

)

Acetic Acid yield

(µmol g-1

h-1

)

P25 20.4 1.79 8.91

TiO2 0 0 -

GOT 22.4 50.4 3.09

TiO2_CuCl2 14.9 2.18 5.71

TiO2_Cu2(Ac) 17.7 15.4 0.710

TiO2_Cu(NO3)2 23.9 124 0.870

TiO2_Cu2O 0 0 -

GOT_CuCl2 0 0 4.74

GOT_Cu2(Ac) 22.8 12.7 8.55

GOT_Cu(NO3)2 22.1 27.7 1.40

GOT_Cu2O 0 0 -

4.3. Mechanism of reaction

For the photocatalytic reduction of CO2 with H2O over semiconductors materials,

the reaction pathway generally includes the following steps: the adsorption of reactants

on the catalyst; activation of the adsorbed reactants by the photogenerated charge

carriers; formation of surface intermediates; conversion of intermediates to products;

desorption of products from catalyst surface; and regeneration of the catalyst. Each of

these steps determines the dynamics of the reaction process and affects the final

products from CO2 conversion [27, 37].

Under the conditions used in this work, i.e alkaline solution (0.2 M NaOH), the

possibility of more hydroxyl ions serves as hole scavengers which reduces the

recombination of holes and electrons [65].

The consideration in selecting copper oxide is based on its potential redox value,

which represents their ability to attack electrons. In this case, Cu+ specie has been

reported to be more active than Cu2+

and metallic Cu0, because Cu

+ has the highest

positive potential redox value (Cu+/Cu

0 = 0.52 V). Therefore, Cu2O should effectively

act as an electron trapper to inhibit electron-hole recombination [32] .


Various reaction schemes for the photocatalytic reduction of CO2 with H2O on

different catalysts have been proposed in literature [4, 64, 65, 68]. In the present study,

methanol, ethanol and acetic acid were the three major identified products obtained

from CO2 photoreduction, although other product as formic acid and oxalic acid, among

others, were also detected. Figure 16 shows a mechanism route proposed for the

photocatalytic reduction of CO2. The formation of methanol and ethanol can be

explained by the electrochemical Eq. 8 and Eq. 10. Moreover, the oxalic acid formation

as well as the acetic acid formation are described in Eq. 12-13 and Eq. 14, respectively.

Figure 16 - Schematic diagram showing mechanism of CO2 photoreduction on Cu-loaded

photocatalyst. Reprinted from [65].

With respect to the route of CH3OH formation, the CO2 photoreduction

mechanism in the presence of water starts with the adsorption of both reactants, CO2

and H2O leading to adsorbed species, followed by their activation by one-electron and

one-hole transfer, respectively. Positive holes (h+) can oxidize water and/or carbonic

acid to produce H+ and HO

• radical on the surface of the catalyst, as depicted in Figure

16, while the CO2 molecule together with the electrons in the conduction band (e-)

results in the formation of the CO2 radical anion (

2CO ) according to Eq. 4.

The

2CO radical anion reacts with protons and electrons forming unstable

•COOH species (Eq. 12). These chemical species has two possible ways to achieve the

stability. On one hand, the coupling of two similar •COOH species gives rise to the

formation of oxalic acid (Eq. 12-13). The formation of formic (HCOOH) and/or oxalic


acid (HOOC-COOH) as primary products, has been previously reported by monitoring

the FTIR spectra of the solid photocatalyst [70].

Oxalic Acid formation:

COOHHCO 2 (12)

422 OHCCOOHCOOH

(13)

Acetic acid formation:

OHCOOHCHHCOOHOHCH 233

(14)


5. Conclusions, future work and final remarks

5.1. Conclusions

Graphene oxide (GO)-TiO2 (GOT) composite and Cu-loaded (2 wt.%) GOT

composites were prepared using different Cu-precursors upon thermal treatment (500

ºC).

The presence of GO and copper in the composites increased the absorption in the

visible spectral range, enhancing the CO2 photoreduction in aqueous phase, with

formation of methanol and ethanol, as main products under near-UV/Vis light

irradiation.

High methanol yield occurred with the GOT composite comprising 4 wt.% of GO,

which was attributed to the optimal self-assembly between GO and TiO2 particles, GO

acting as electron acceptor and donor.

Among all the prepared Cu-loaded composites, TiO2_Cu(NO3)2 was the best

material for both methanol and ethanol production. This effect could be related with Cu

(I) species acting as an effective electron trapper and able to inhibit the recombination

of electron-hole pairs. The largest amount of acetic acid was produced by P25 under

near-UV/Vis light irradiation (360 min) probably due to its high catalytic activity and

low selectivity.

The lowest photocatalytic activities were obtaining using CuCl2 and Cu2O as

copper precursor, due to organic residues or inorganic ions that may still be present on

the catalyst surface even after calcination.

The mechanism of reaction was based on the formation of •COOH species that

react with the protons generated to the formation of methanol, ethanol and acetic acid,

although mechanism study is still in progress to confirm this suggestion.

In general, it seems that the selectivity towards methanol formation arises from

the presence of Cu on the photocatalysts, while the visible light photoresponse would be

introduced by the presence of GO material.


5.2. Future work

The conversion of CO2 by photocatalysis is seen nowadays as one of the most

promising approaches to solve CO2 footprint problems, since CO2 can be reduced to

useful compounds by irradiation with solar light. To improve the photocatalytic

performance of the materials presented in this MSc Thesis, the following possible future

studies are proposed:

- To prepare photocatalysts with the impregnation method over the photocatalysts

(TiO2 and GOT), using Cu(NO3)2 as Cu precursor;

-To add other type of co-catalysts (such as Pt or Au) to the semiconductors used

(TiO2 and GOT);

-To improve the thermal treatment conditions of the catalysts in order to enhance

their photocatalytic properties;

-To increase the time of the photocatalytic reactions in order to reach a better

understanding of the evolution of the products over time;

-To use different light sources, such as LEDs;

-To reuse the catalysts in consecutive runs and analyze their photocatalytic

behaviour.

5.3. Final remarks

This research was completed with much satisfaction since it is focused on an

extremely interesting technological solution, where CO2 is used as a feedstock for the

chemical industry and transformed into a chemical valuable product.

Throughout the development of this work, the new findings can contribute to the

scientific knowledge by improving and making this technology more viable in a near

future. At the moment, every advance in this area is very relevant.

This research was an enriching experience at the academic level because it

allowed me to understand new concepts, related to the field of environmental and

chemical engineering. This study was operated in a laboratory unit of excellence, where

I overcame challenges and obstacles that allowed me to develop technical skills, which

could be useful in my future.


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Appendix A: Calibration curves of methanol, ethanol and acetic acid

Figure A1 - Calibration curve of methanol.

Figure A2 - Calibration curve of ethanol.

y = 16850x - 1256.4R² = 0.993

0

20000

40000

60000

80000

100000

0 1 2 3 4 5

Are

a

Methanol concentration (mM)

y = 53082x + 2277.1R² = 0.9948

0

50000

100000

150000

200000

250000

300000

0 1 2 3 4 5

Are

a

Ethanol concentration (mM)


Figure A3 - Calibration curve of acetic acid.

y = 842228x - 17931R² = 0.9995

0

250000

500000

750000

1000000

1250000

1500000

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Are

a

Acetic Acid concentration (mM)

co2 conversion to renewable fuels · integrated masters in environmental engineering 2014/2015 co 2...

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